Physics Letters B 737 (2014) 162–166
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
3.55 keV X-ray line signal from ex cited dark matter
in
radiative neutrino model
Hiroshi Okada
a,∗
, Takashi Toma
b
a
School of Physics, KIAS, Seoul 130-722, Republic of Korea
b
Institute for Particle Physics Phenomenology, University of Durham, Durham DH1 3LE, United Kingdom
a r t i c l e i n f o a b s t r a c t
Article history:
Received
27 April 2014
Received
in revised form 29 July 2014
Accepted
19 August 2014
Available
online 22 August 2014
Editor:
J. Hisano
We study an exciting dark matter scenario in a radiative neutrino model to explain the X-ray line signal
at 3.55 keV recently reported by XMN-Newton X-ray observatory using data of various galaxy clusters
and Andromeda galaxy. We show that the required large cross section for the up-scattering process to
explain the X-ray line can be obtained via the resonance of the pseudo-scalar. Moreover, this model can
be compatible with the thermal production of dark matter and the constraint from the direct detection
experiment.
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/3.0/). Funded by SCOAP
3
.
1. Introduction
In the light of anomalous X-ray line signal at 3.55 keV from
the analysis of XMN-Newton X-ray observatory data of various
galaxy clusters and Andromeda galaxy [1,2], dark matter (DM)
whose mass is in the range from keV to GeV comes into one of
the promising candidates. Subsequently, a number of literatures re-
cently
arose around the subject [3–22]. As for the keV scale DM,
for example, a sterile neutrino can be one of the typical candidates
to explain the X-ray anomaly that requires tiny mixing between
the DM and the active neutrino; sin
2
2θ ≈ 10
−10
[1]. However,
these
scenarios suggest that neutrino masses cannot be derived
consistently with the sterile neutrino DM due to its too small mix-
ing.
Moreover, the sterile neutrino DM mass is out of the range in
the direct detection searches such as LUX [23], which is currently
the most powerful experiment to constrain the kind of Weakly In-
teracting
Massive Particle.
As
for the GeV scale DM, on the other hand, the exciting DM
scenario which requests a pair of ground state and excited DM
is known to explain the X-ray [7]. In this framework, the emis-
sion
of X-ray is simply realized as follows. After the ground state
DM up-annihilates into the excited DM pair, it can decay into pho-
tons
(X-ray) and the ground state DM. The mass difference among
them is assumed to be the energy of the X-ray, 3.55 keV. Since the
framework of the exciting DM is simple, this scenario can be appli-
cable
to various models such as radiative neutrino models [24–27].
*
Corresponding author.
E-mail
addresses: hokada@kias.re.kr (H. Okada), takashi.toma@durham.ac.uk
(T. Toma).
Table 1
The
new particle contents and the charges for bosons where i = 1–3 is generation
index.
Particle L
i
e
i
N
i
η ΦΣ
(SU(2)
L
, U (1)
Y
)(2, −1/2)(1, −1)(1, 0)(2, 1/2)(2, 1/2)(1,0)
Z
3
ω
2
1 ωω
2
ω
2
ω
Z
2
++−−++
In this kind of models, small neutrino masses and existence of DM
would be accommodated unlike the sterile neutrino DM scenar-
ios
above. Moreover, the DM can be testable in direct detection
searches because the DM mass is GeV scale.
In
this Letter, we account for the X-ray anomaly in terms of
an excited DM scenario in a simple extended model with radia-
tive
neutrino masses [25], in which three right-handed neutrinos,
a SU(2)
L
doublet scalar and a singlet scalar are added to the Stan-
dard
Model (SM) and the first two lightest right-handed neutrinos
are assumed to be a pair of ground state and excited state DM.
2. The model
2.1. Model setup
The particle contents and charge assignments of the model we
consider are shown in Table 1. We introduce three right-handed
neutrinos N
i
(i = 1–3) where the first two lightest ones are iden-
tified
to be a pair of ground state and excited state DM. We
also introduce a SU(2)
L
doublet inert scalar η that is assumed
not to have vacuum expectation value (VEV), and a gauge sin-
glet
boson Σ with non-zero VEV in addition to the SM like Higgs
boson Φ. The Z
2
symmetry is imposed to assure the stability
http://dx.doi.org/10.1016/j.physletb.2014.08.046
0370-2693/
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). Funded by
SCOAP
3
.
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